Computer systems may have main memory operated in a mirrored mode, meaning that two sets of main memory contents may be maintained within the computer. Should one of the sets of main memory contents fail as indicated, for example, by an uncorrectable error, the computer system may continue operating using the mirrored or second set, rather than experiencing a computer system failure. Computer systems operating mirrored main memory may implement the mirroring feature across two independent memory boards. Thus, if memory on one memory board experiences a failure, the computer system may continue to operate (in a non-mirrored state) using the remaining functional memory board while the failed memory board may be removed, repaired and/or replaced.
After initial insertion of a new or repaired memory board in a computer system designed to operate in mirrored mode, the newly inserted board may not have a complete copy of the main memory contents. The computer system may copy the data and programs, on a location-by-location basis, from the active memory board to a corresponding location in the new memory board. Because of the location-by-location copying, it may be necessary to operate main memory boards having the same amount of memory. Expanding the amount of main memory in a computer system designed to operate mirrored main memory may therefore require powering down the computer system and adding memory to the memory boards. Thus, it may not be possible to add main memory to a computer system while in operation, because at some point one of the memory boards may have a different amount of memory.